Which of these numbers is prime? ${3,\ 45,\ 51,\ 72,\ 81}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 51 are 1, 3, 17, and 51. The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 3 is a prime number.